ci/vs2019: ignore _build and _install directory by write * into .gitignore

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16934>

.gitlab-ci/windows/mesa_build.ps1

@@ -22,6 +22,9 @@ Remove-Item -Recurse -Force $installdir
 New-Item -ItemType Directory -Path $builddir
 New-Item -ItemType Directory -Path $installdir
 
+Write-Output "*" > $builddir\.gitignore
+Write-Output "*" > $installdir\.gitignore
+
 Write-Output builddir:$builddir
 Write-Output installdir:$installdir
 Write-Output sourcedir:$sourcedir